Method for Forming a Polyarylene Sulfide

ABSTRACT

A method for forming a polyarylene sulfide with a relatively low content of volatile malodorous compounds is provided. More particularly, such low compound levels may be achieved by subjecting a washed polyarylene sulfide to a heat treatment in the presence of an atmosphere that includes an inert gas, wherein the heat treatment occurs at a temperature of from about 150° C. to about 275° C.

RELATED APPLICATION

The present application claims priority to U.S. Provisional ApplicationSer. No. 62/951,096, filed on Dec. 20, 2019, which is incorporatedherein in its entirety by reference thereto.

BACKGROUND OF THE INVENTION

Polyarylene sulfides are high-performance polymers that may withstandhigh thermal, chemical, and mechanical stresses and are beneficiallyutilized in a wide variety of applications. Polyarylene sulfides aregenerally formed via polymerization of a dihaloaromatic monomer with analkali metal sulfide or an alkali metal hydrosulfide in an organic amidesolvent. Following formation, the polyarylene sulfide is washed toseparate the polymer from the solvent, unreacted monomers, and otherimpurities. Many conventional washing solutions rely on the use ofacetone to quickly remove the reaction solvent and a substantial portionof partially polymerized oligomers from the polyarylene sulfide.Unfortunately, polyarylene sulfides that are washed with such solutionstend to contain residual amounts of certain malodorous compounds, suchas mesityl oxide (“MO”), mercapto-4-methylpentan-2-one (“MMP”), and/orbutyrolactone (“BL”), which are formed when acetone and NMP arecontacted. As such, a need currently exists for polyarylene sulfide witha low level of malodorous compounds and for improved processes forforming polyarylene sulfides.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method forforming a polyarylene sulfide is disclosed. The method comprisessubjecting a polyarylene sulfide to a washing cycle to form a washedpolyarylene sulfide, and thereafter subjecting the washed polyarylenesulfide to a heat treatment in the presence of an atmosphere thatincludes an inert gas. The heat treatment occurs at a temperature offrom about 150° C. to about 275° C.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE FIGURES

The present disclosure may be better understood with reference to thefollowing figures:

FIG. 1 illustrates one embodiment of a sedimentation column that may beemployed in the present invention;

FIG. 2 illustrates a cross-sectional top view of the middle section ofthe sedimentation column of FIG. 1 at the slurry inlet; and

FIG. 3 illustrates several different embodiments of longitudinalcross-sectional shapes of a middle section of a sedimentation columnthat may be employed in the present invention.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

Generally speaking, the present invention is directed to a method forforming a polyarylene sulfide with a relatively low content of volatilecompounds, which specifically includes mesityl oxide (“MO”),mercapto-4-methylpentan-2-one (“MMP”), butyrolactone (“BL”), diacetonealcohol (“DAA”), thiophenol (“PhSH”), p-dichlorobenzene (“pDCB”),methylthiophenol (“MeSPh”), and/or N-methylpyrrolidone (“NMP”). Moreparticularly, the polyarylene sulfide of the present invention may havea total volatile content of about 175 parts per million (“ppm”) or less,in some embodiments about 100 ppm or less, and in some embodiments, from0 to about 70 ppm. Such compounds may either have an unpleasant odor orare precursors to other compounds that have an unpleasant odor. Forinstance, the the polyarylene sulfide may have an MMP content of about75 ppm or less, in some embodiments about 60 ppm or less, in someembodiments about 50 ppm or less, and in some embodiments, from 0 toabout 25 ppm. The polyarylene sulfide may have an MO content of about 75ppm or less, in some embodiments about 60 ppm or less, in someembodiments about 50 ppm or less, and in some embodiments, from 0 toabout 25 ppm. The polyarylene sulfide may also have a BL content ofabout 65 ppm or less, in some embodiments about 55 ppm or less, in someembodiments about 40 ppm or less, and in some embodiments, from 0 toabout 35 ppm. Likewise, the polyarylene sulfide may an NMP content ofabout 20 ppm or less, in some embodiments about 15 ppm or less, and insome embodiments, from 0 to about 10 ppm, as well as a pDCB content ofabout 4 ppm or less, in some embodiments about 3.5 ppm or less, and insome embodiments, from 0 to about 3 ppm. The present inventor hasdiscovered that such low volatile levels may be achieved by selectivelycontrolling the manner in which the polyarylene sulfide is processedafter it is washed.

More particularly, the process includes subjecting the polyarylenesulfide to a heat treatment at a relatively high temperature of fromabout 150° C. to about 275° C., in some embodiments from about 180° C.to about 270° C., and in some embodiments, from about 225° C. to about265° C. The heat treatment may occur in one or multiple steps and may beramped up to the temperature noted above, or simply held constant at thedesired temperature during the entire treatment. Regardless, theduration of the heat treatment is typically from about 0.1 to about 15hours, in some embodiments from about 0.5 to about 10 hours, and in someembodiments, from about 1 to about 5 hours. The heat treatment alsotypically occurs at or near atmospheric pressure, such as at a pressureof about 0.6 to about 1.2 atm, and in some embodiments, from about 0.8to about 1.2 atm.

To help ensure that the heat treatment does not adversely impact thecolor of the polyarylene sulfide, the heat treatment is generallyconducted in the presence of an inert gas. The inert gas may include,for instance, nitrogen, helium, argon, xenon, neon, krypton, radon, andso forth, as well as mixtures thereof. Typically, inert gases constitutethe substantial majority of the atmosphere within the housing, such asfrom about 50 wt. % to 100 wt. %, in some embodiments from about 75 wt.% to 100 wt. %, and in some embodiments, from about 90 wt. % to 100 wt.% of the atmosphere (e.g., 100 wt. %). If desired, a relatively smallamount of non-inert gases may also be employed, such as carbon dioxide,oxygen, water vapor, etc. In such cases, however, the non-inert gasestypically constitute 10 wt. % or less, in some embodiments 5 wt. % orless, in some embodiments about 2 wt. % or less, in some embodimentsabout 1 wt. % or less, and in some embodiments, from about 0.01 wt. % toabout 1 wt. % of the atmosphere. The resulting Yellowness Index of thepolyarylene sulfide after heat treatment may remain relatively low, suchas about 25 or less, in some embodiments about 15 or less, in someembodiments about 10 or less, and in some embodiments, from about 0.1 toabout 9.5, as determined in accordance with ASTM E313-15 (illuminantD65, 10 degree observer). The purity of the resulting polyarylenesulfide may likewise be relatively high, such as about 90% or more, insome embodiments about 95% or more, and in some embodiments, or about98% or more.

Through the process described above, the present inventor has alsodiscovered that the polyarylene sulfide can retain a relatively higholigomer content, which in turn, helps minimize the melt viscosity. Theoligomer content may, for instance, range from about 0.5 wt. % to about2 wt. %, in some embodiments from about 0.8 wt. % to about 1.8 wt. %,and in some embodiments, from about 1.2 wt. % to about 1.6 wt. %. Thepolyarylene sulfide may likewise have a melt viscosity of about 4,000poise or less, in some embodiments about 2,500 poise or less, and insome embodiments, from about 100 to about 2,000 poise, as determined inaccordance with ISO Test No. 11443:2005 at a shear rate of 1,200 s⁻¹ andat a temperature of 310° C. In addition, the crystallization temperatureof the polyarylene sulfide may also remain relatively low, such as about240° C. or less, in some embodiments about 230° C. or less, and in someembodiments, from about 180° C. to about 225° C. The weight averagemolecular weight of the polyarylene sulfide may also be from about10,000 to about 120,000 Daltons, in some embodiments about 15,000Daltons to about 110,000 Daltons, and in some embodiments, from about20,000 to about 100,000 Daltons. The polydispersity index (weightaverage molecular weight divided by the number average molecular weight)may be relatively low, thus resulting in a polymer that is more readilyformed into particles with a narrow particle size distribution. Forinstance, the polydispersity index of the polyarylene sulfide may beabout 4.3 or less, in some embodiments about 4.1 or less, and in someembodiments, from about 2.0 to about 4.0. The number average molecularweight of the polyarylene sulfide may, for instance, be from about 5,000about 80,000 Daltons, in some embodiments about 10,000 Daltons to about70,000 Daltons, and in some embodiments, from about 20,000 to about60,000 Daltons.

Various embodiments of the present invention will now be described inmore detail below.

I. Polyarylene Sulfide

The polyarylene sulfide generally has repeating units of the formula:

—[(Ar¹)_(n)—X]_(m)—[(Ar²)_(i)—Y]_(j)—[(Ar³)_(k)—Z]_(l)—[(Ar⁴)_(o)—W]_(p—)

wherein,

Ar¹, Ar², Ar³, and Ar⁴ are independently arylene units of 6 to 18 carbonatoms;

W, X, Y, and Z are independently bivalent linking groups selected from—SO₂—, —S—, —SO—, —CO—, —O—, —C(O)O— or alkylene or alkylidene groups of1 to 6 carbon atoms, wherein at least one of the linking groups is —S—;and

n, m, i, j, k, l, o, and p are independently 0, 1, 2, 3, or 4, subjectto the proviso that their sum total is not less than 2.

The arylene units Ar¹, Ar², Ar³, and Ar⁴ may be selectively substitutedor unsubstituted. Advantageous arylene units are phenylene, biphenylene,naphthylene, anthracene and phenanthrene. The polyarylene sulfidetypically includes more than about 30 mol %, more than about 50 mol %,or more than about 70 mol % arylene sulfide (—S—) units. For example,the polyarylene sulfide may include at least 85 mol % sulfide linkagesattached directly to two aromatic rings. In one particular embodiment,the polyarylene sulfide is a polyphenylene sulfide, defined herein ascontaining the phenylene sulfide structure —(C₆H₄—S)_(n)— (wherein n isan integer of 1 or more) as a component thereof.

The polyarylene sulfide may be a homopolymer or copolymer. For instance,selective combination of dihaloaromatic compounds may result in apolyarylene sulfide copolymer containing not less than two differentunits. For instance, when p-dichlorobenzene is used in combination withm-dichlorobenzene or 4,4′-dichlorodiphenylsulfone, a polyarylene sulfidecopolymer may be formed containing segments having the structure offormula:

and segments having the structure of formula:

or segments having the structure of formula:

The polyarylene sulfide(s) may be linear, semi-linear, branched orcrosslinked. Linear polyarylene sulfides typically contain 80 mol % ormore of the repeating unit —(Ar—S)—. Such linear polymers may alsoinclude a small amount of a branching unit or a cross-linking unit, butthe amount of branching or cross-linking units is typically less thanabout 1 mol % of the total monomer units of the polyarylene sulfide. Alinear polyarylene sulfide polymer may be a random copolymer or a blockcopolymer containing the above-mentioned repeating unit. Semi-linearpolyarylene sulfides may likewise have a cross-linking structure or abranched structure introduced into the polymer a small amount of one ormore monomers having three or more reactive functional groups.

Various techniques may generally be employed to synthesize thepolyarylene sulfide. By way of example, a process for producing apolyarylene sulfide may include reacting a material that provides ahydrosulfide ion (e.g., an alkali metal sulfide) with a dihaloaromaticcompound in an organic amide solvent. The alkali metal sulfide may be,for example, lithium sulfide, sodium sulfide, potassium sulfide,rubidium sulfide, cesium sulfide or a mixture thereof. When the alkalimetal sulfide is a hydrate or an aqueous mixture, the alkali metalsulfide may be processed according to a dehydrating operation in advanceof the polymerization reaction. An alkali metal sulfide may also begenerated in situ. In addition, a small amount of an alkali metalhydroxide may be included in the reaction to remove or react impurities(e.g., to change such impurities to harmless materials) such as analkali metal polysulfide or an alkali metal thiosulfate, which may bepresent in a very small amount with the alkali metal sulfide.

The dihaloaromatic compound may be, without limitation, ano-dihalobenzene, m-dihalobenzene, p-dihalobenzene, dihalotoluene,dihalonaphthalene, methoxy-dihalobenzene, dihalobiphenyl, dihalobenzoicacid, dihalodiphenyl ether, dihalodiphenyl sulfone, dihalodiphenylsulfoxide or dihalodiphenyl ketone. Dihaloaromatic compounds may be usedeither singly or in any combination thereof. Specific exemplarydihaloaromatic compounds may include, without limitation,p-dichlorobenzene; m-dichlorobenzene; o-dichlorobenzene;2,5-dichlorotoluene; 1,4-dibromobenzene, 1,4-dichloronaphthalene;1-methoxy-2,5-dichlorobenzene; 4,4′-dichlorobiphenyl;3,5-dichlorobenzoic acid; 4,4′-dichlorodiphenyl ether;4,4′-dichlorodiphenylsulfone; 4,4′-dichlorodiphenylsulfoxide; and4,4′-dichlorodiphenyl ketone. The halogen atom may be fluorine,chlorine, bromine or iodine, and two halogen atoms in the samedihalo-aromatic compound may be the same or different from each other.In one embodiment, o-dichlorobenzene, m-dichlorobenzene,p-dichlorobenzene or a mixture of two or more compounds thereof is usedas the dihalo-aromatic compound. As is known in the art, it is alsopossible to use a monohalo compound (not necessarily an aromaticcompound) in combination with the dihaloaromatic compound in order toform end groups of the polyarylene sulfide or to regulate thepolymerization reaction and/or the molecular weight of the polyarylenesulfide.

Although by no means required, the polyarylene sulfide can, in certainembodiments be formed in a multi-stage process that includes at leasttwo separate formation stages. One stage of the formation process mayinclude reaction of a complex that includes a hydrolysis product of anorganic amide solvent and alkali metal hydrogen sulfide with adihaloaromatic monomer to form a prepolymer. Another stage of theprocess may include further polymerization of the prepolymer to form thefinal product. Optionally, the process may include yet another stage inwhich the organic amide solvent and an alkali metal sulfide are reactedto form the complex. The different stages may take place in a singlereactor or in separate reactors.

In one embodiment, for instance, a multi-stage process may be employedin which a first stage of the process includes the reaction of anorganic amide solvent and an alkali metal sulfide within a reactor toform a complex that includes a hydrolysis product of the organic amidesolvent (e.g., an alkali metal organic amine carboxylic acid salt) andan alkali metal hydrosulfide. Exemplary organic amide solvents as may beused in a forming the polyarylene sulfide may include, withoutlimitation, N-methyl-2-pyrrolidone; N-ethyl-2-pyrrolidone;N,N-dimethylformamide, N,N-dimethylacetamide, N-methylcaprolactam,tetramethylurea; dimethylimidazolidinone; hexamethyl phosphoric acidtriamide and mixtures thereof. The alkali metal sulfide may be, forexample, lithium sulfide, sodium sulfide, potassium sulfide, rubidiumsulfide, cesium sulfide or a mixture thereof. An alkali metal sulfidemay also be generated in situ. For instance, a sodium sulfide hydratemay be prepared within the first reactor from sodium hydrogen sulfideand sodium hydroxide that may be fed to the reactor. When a combinationof alkali metal hydrogen sulfide and alkali metal hydroxide are fed tothe reactor to form the alkali metal sulfide, the molar ratio of alkalimetal hydroxide to alkali metal hydrogen sulfide may be between about0.80 and about 1.50. In addition, a small amount of an alkali metalhydroxide may be included in the first reactor to remove or reactimpurities (e.g., to change such impurities to harmless materials) suchas an alkali metal polysulfide or an alkali metal thiosulfate, which maybe present in a very small amount with the alkali metal sulfide.

The feed to the reactor may include sodium sulfide (Na₂S) (which may bein the hydrate form), N-methyl-2-pyrrolidone (NMP) and water. Reactionbetween the water, sodium sulfide and the NMP may form a complexincluding sodium methylaminobutyrate (SMAB—a hydrolysis product of NMP)and sodium hydrogen sulfide (NaSH) (SMAB-NaSH) according to thefollowing reaction scheme:

According to one embodiment, a stoichiometric excess of the alkali metalsulfide may be utilized, though this is not a requirement of theformation stage. For instance, the molar ratio of organic amide solventto sulfur in the feed may be from 2 to about 10, or from about 3 toabout 5, and the molar ratio of water to the sulfur source in the feedmay be from about 0.5 to about 4, or from about 1.5 to about 3.

During the formation of the complex, the pressure within the reactor maybe held at or near atmospheric pressure. To maintain the low pressurereaction conditions, vapor may be removed from the reactor. The mainconstituents of the vapor may include water and hydrogen sulfideby-product. Hydrogen sulfide of the vapor can, for instance, beseparated at a condenser. A portion of the water that is separated atsuch a condenser may be returned to the reactor to maintain the reactionconditions. Another portion of the water may be removed from the processso as to dehydrate the SMAB-NaSH solution formed in the first stage. Forinstance, the molar ratio of water to NaSH (or the ratio of oxygen tosulfur) in the product solution of the first reactor may less than about1.5, or may be between about 0.1 and about 1 such that the SMAB-NaSHcomplex solution that is fed to the second stage reactor isnear-anhydrous.

Once formed, the SMAB-NaSH complex may then be reacted with adihaloaromatic monomer (e.g., p-dichlorobenzene) and a suitable solventso as to form the polyarylene sulfide prepolymer in the second stage ofthe process. This may occur in the same or a different reactor as thefirst stage of the process. The amount of the dihaloaromatic monomer(s)per mole of the effective amount of the charged alkali metal sulfide maygenerally be from about 1.0 to about 2.0 moles, in some embodiments fromabout 1.05 to about 2.0 moles, and in some embodiments, from about 1.1to about 1.7 moles. If desired, a relatively low molar ratio of thedihaloaromatic monomer to the alkali metal hydrogen sulfide of thecomplex may be employed. For instance, the ratio of the dihaloaromaticmonomer to sulfur may be from about 0.8 to about 1.5, and in someembodiments, from about 1.0 to about 1.2. The relatively low ratio ofthe dihaloaromatic monomer to the alkali metal hydrogen sulfide of thecomplex may be favorable for the formation of the final high molecularweight polymer via the condensation polymerization reaction. The ratioof solvent to sulfur in the second stage may also be relatively low. Forinstance, the ratio of the alkali metal hydrogen sulfide of the complexto the organic amide solvent in the second stage (including any solventadded and remaining in the complex solution) may be from about 2 toabout 2.5. This relatively low ratio may increase the concentration ofreactants, which may increase the relative polymerization rate and theper volume polymer production rate.

The second stage polymerization reaction may generally be carried out ata temperature of from about 200° C. to about 280° C., or from about 235°C. to about 260° C. The duration of the second stage may be, e.g., fromabout 0.5 to about 15 hours, or from about 1 to about 5 hours. Followingthe second stage polymerization reaction, the mean molar mass of theprepolymer as expressed by the weight average molecular weight, M_(w),may be from about 500 g/mol to about 30,000 g/mol, from about 1000 g/molto about 20,000 g/mol, or from about 2000 g/mol to about 15,000 g/mol.

The product of the second stage may include the prepolymer, the solvent,and one or more salts that are formed as a by-product of thepolymerization reaction. For example, the proportion by volume of theprepolymer solution of salt that is formed as a byproduct to thereaction may be from about 0.05 to about 0.25, or from about 0.1 toabout 0.2. Salts included in the reaction mixture may include thoseformed as a byproduct during the reaction as well as other salts addedto the reaction mixture, for instance as a reaction promoter. The saltsmay be organic or inorganic, i.e., may consist of any combination oforganic or inorganic cations with organic or inorganic anions. They maybe at least partially insoluble in the reaction medium and have adensity different from that of the liquid reaction mixture. According toone embodiment, at least a portion of the salts in the prepolymermixture formed during the second stage may be removed from the mixture.For instance, the salts may be removed by use of screens or sieves ashas been utilized in traditional separation processes. A salt/liquidextraction process may alternatively or additionally be utilized inseparating the salt from the prepolymer solution. In one embodiment, ahot filtration process may be utilized in which the solution may befiltered at a temperature at which the prepolymer is in solution and thesalts are in the solid phase. According to one embodiment, a saltseparation process may remove about 95% or more of the salts includingin the prepolymer solution formed during the second stage. For instance,greater than about 99% of the salts may be removed from the prepolymersolution.

Following the prepolymer polymerization reaction in the second stage ofthe process and the filtration process, an optional third stage of theformation may take place during which the molecular weight of theprepolymer is increased. Again, this stage may occur in the same or adifferent reactor from the first and second stages. The reactantsemployed during this stage may include the prepolymer solution from thesecond stage, solvent, one or more dihaloaromatic monomers, and asulfur-containing monomer. For instance, the amount of thesulfur-containing monomer added in third stage may be about 10% or lessof the total amount required to form the product polyarylene sulfide. Inthe illustrated embodiment, the sulfur-containing monomer is sodiumsulfide, but this is not a requirement of the third stage, and othersulfur containing monomers, such as an alkali metal hydrogen sulfidemonomer may alternatively be utilized.

The third reaction conditions may be nearly anhydrous, with the ratio ofwater to the sulfur-containing monomer less than about 0.2, for instancebetween 0 and about 0.2. The low water content during the third stage ofthe process may increase the polymerization rate and the polymer yieldas well as reduce formation of undesired side reaction by-products asthe conditions are favorable for nucleophilic aromatic substitution, asdiscussed above. Moreover, as pressure increases in the third stage aregenerally due to water vaporization, low water content during this stagemay allow the third reaction to be carried out at a constant, relativelylow pressure, for instance less than about 1500 kPa.

The reaction conditions during the third stage may also include arelatively low molar ratio for the solvent to the sulfur-containingmonomer. For instance, the ratio of solvent to sulfur-containing monomermay be from about 2 to about 4, or from about 2.5 to about 3. Thereaction mixture of the third stage may be heated to a temperature offrom about 120° C. to about 280° C., or from about 200° C. to about 260°C. and the polymerization may continue until the melt viscosity of thethus formed polymer is raised to the desired final level. The durationof the second polymerization step may be, e.g., from about 0.5 to about20 hours, or from about 1 to about 10 hours. The weight averagemolecular weight of the formed polyarylene sulfide may vary as is known,but in one embodiment may be from about 1000 g/mol to about 500,000g/mol, from about 2,000 g/mol to about 300,000 g/mol, or from about3,000 g/mol to about 100,000 g/mol. Following the third stage, and anydesired post-formation processing, the polyarylene sulfide may bedischarged from the reactor, typically through an extrusion orificefitted with a die of desired configuration, cooled, and collected.Commonly, the polyarylene sulfide may be discharged through a perforateddie to form strands that are taken up in a water bath, pelletized anddried. The polyarylene sulfide may also be in the form of a strand,granule, or powder.

II. Washing Cycle

Once formed, the polyarylene sulfide may be subjected to a washing cyclethat contacting the polyarylene sulfide with a washing solution, whichtypically includes water and/or an organic solvent. When employed, theorganic solvent is typically an aprotic solvent. Particularly suitableaprotic organic solvents include, for instance, halogen-containingsolvents (e.g., methylene chloride, 1-chlorobutane, chlorobenzene,1,1-dichloroethane, 1,2-dichloroethane, chloroform, and1,1,2,2-tetrachloroethane); ether solvents (e.g., diethyl ether,tetrahydrofuran, and 1,4-dioxane), ketone solvents (e.g., acetone andcyclohexanone); ester solvents (e.g., ethyl acetate); lactone solvents(e.g., butyrolactone); carbonate solvents (e.g., ethylene carbonate andpropylene carbonate); amine solvents (e.g., triethylamine and pyridine);nitrile solvents (e.g., acetonitrile and succinonitrile); amide solvents(e.g., N,N′-dimethylformamide, N,N′-dimethylacetamide, tetramethylureaand N-methylpyrrolidone); nitro-containing solvents (e.g., nitromethaneand nitrobenzene); sulfide solvents (e.g., dimethylsulfoxide andsulfolane); and so forth. Of, course, other types of organic solventsmay also be employed. In certain embodiments, for instance, proticsolvents may be employed, such as glycols (e.g., propylene glycol,butylene glycol, triethylene glycol, hexylene glycol, polyethyleneglycols, ethoxydiglycol, and dipropyleneglycol); carboxylic acids (e.g.,formic acid); alcohols (e.g., methanol, ethanol, n-propanol,iso-propanol, and butanol); and so forth. Particularly suitable proticsolvents are aliphatic alcohols, such as ethanol, propanol, methanol,isopropanol, butanol, and so forth.

The washing cycle may include one or more washing stages in which thepolyarylene sulfide is contacted with a washing solution. In oneparticular embodiment, for instance, the washing cycle may include afirst washing stage in which the polyarylene sulfide is contacted with afirst washing solution that contains an organic solvent as a principalcomponent. The first washing stage may include one or multiple distinctwashing steps. For example, the first washing stage may include from 1to 10, in some embodiments from 1 to 6, and in some embodiments, from 2to 4 distinct steps in which a first washing solution is contacted withthe polyarylene sulfide. During this stage, the ratio of solids (e.g.,polymer) to liquids typically ranges from about 1 to about 10, and someembodiments, from about 2 to about 8. The first washing stage may alsobe generally conducted for a time period of from about 1 minute to about500 minutes, and in some embodiments, from about 5 minutes to about 360minutes. Regardless, the first washing solution generally contains theorganic solvent in an amount of about 50 wt. % or more, in someembodiments about 60 wt. % or more, in some embodiments about 75 wt. %or more, and in some embodiments, from about 85 wt. % to 100 wt. % ofthe first washing solution. Through the use of such a “solvent-rich”washing solution, organic impurities can be readily removed that wouldotherwise produce odor and can minimize the extent to which oligomersare removed. In certain cases, it may be desirable to employ a firstwashing solution that contains organic solvents in an amount of about 95wt. % or more (e.g., 100 wt. %). In other embodiments, however, thefirst washing solution may also contain water to help minimize theextent that the organic solvent inadvertently removes shorter polymerchains. When such a mixture is employed, the organic solvent typicallyconstitutes from about 50 wt. % to about 99 wt. %, in some embodimentsfrom about 60 wt. % to 98 wt. %, in some embodiments from about 75 wt. %to about 96 wt. %, and in some embodiments, from about 85 wt. % to about95 wt. % of the first washing solution, while water typicallyconstitutes from about 1 wt. % to about 50 wt. %, in some embodimentsfrom about 2 wt. % to 40 wt. %, in some embodiments from about 4 wt. %to about 25 wt. %, and in some embodiments, from about 5 wt. % to about15 wt. % of the first washing solution.

After the first washing stage, the polyarylene sulfide may also besubjected to a second washing stage in which the polyarylene sulfide iscontacted with a second washing solution that contains water (e.g.,deionized water, recycled water, etc.) as a primary component. Thesecond washing stage may include one or multiple distinct washing steps.For example, the second washing stage may include from 1 to 20, in someembodiments from 2 to 15, and in some embodiments, from 4 to 10 distinctsteps in which a second washing solution is contacted with thepolyarylene sulfide. During this stage, the ratio of solids (e.g.,polymer) to liquids typically ranges from about 1 to about 10, and someembodiments, from about 2 to about 8. The first washing stage may alsobe generally conducted for a time period of from about 1 minute to about500 minutes, and in some embodiments, from about 5 minutes to about 360minutes. Regardless, the second washing solution generally containswater in an amount of about 50 wt. % or more, in some embodiments about70 wt. % or more, in some embodiments about 80 wt. % or more, and insome embodiments, from about 85 wt. % to about 100 wt. % of the secondwashing solution. In certain cases, it may be desirable to employ asecond washing solution that contains water in an amount of about 95 wt.% or more (e.g., 100 wt. %). In other embodiments, however, the secondwashing solution may also contain an organic solvent if so desired. Whensuch a mixture is employed, the organic solvent typically constitutesfrom about 0.1 wt. % to about 30 wt. %, in some embodiments from about0.2 wt. % to 20 wt. %, and in some embodiments, from about 0.5 wt. % toabout 10 wt. % of the second washing solution, while water typicallyconstitutes from about 70 wt. % to about 99.9 wt. %, in some embodimentsfrom about 80 wt. % to 99.8 wt. %, and in some embodiments, from about90 wt. % to about 99.5 wt. % of the first washing solution. The use ofsuch a “water-rich” washing step helps extract any residual organicsolvents, unreacted compounds, and/or impurities from the polymerwithout causing any excessive removal of short chain polymer molecules.

If desired, both the first and second washing solutions may be generallyfree of acetyl compounds (e.g., acetone and/or acetic acid) in that suchcompounds are present in an amount of no more than about 0.1 wt. %, insome embodiments no more than about 0.05 wt. %, and in some embodiments,and in some embodiments, no more than about 0.01 wt. % of a washingsolution. Without intending to be limited by theory, the use of suchwashing solutions can minimize the extent of odor-producing sidereactions that would otherwise occur in an acetyl-based washing process.Of course, various other suitable materials may also be used in thewashing solutions, such as stabilizers, surfactants, pH modifiers, etc.For example, basic pH modifiers may be employed to help raise the pH ofthe washing solution to the desired level, which is typically above 7,in some embodiments from about 8.0 to about 13.5, in some embodimentsfrom about 9.0 to about 13.5, and in some embodiments, from about 11.0to about 13.0. Suitable basic pH modifiers may include, for instance,ammonia, alkali metal hydroxides (e.g., sodium hydroxide, lithiumhydroxide, potassium hydroxide, etc.), alkaline earth metal hydroxides,etc., as well as combinations thereof. Further, the solutions may alsocontain low levels of impurities (e.g., chloride, sodium, product ofdecomposition of monomer and solvent, etc.), particularly when recycledsolutions are employed.

The use of certain washing temperatures may help improve the purity ofthe resulting polymer, as well as enhance the efficiency of the washingprocess. For example, the second “water-rich” washing solution may be ata temperature of about 90° C. or more, in some embodiments from about90° C. to about 200° C., in some embodiments from about 95° C. to about180° C., and in some embodiments, from about 100° C. to about 160° C.Through the use of such high temperatures for the “water-rich” solution,any residual oligomers may melt so that they can be more readilyextracted. The temperature of the first “solvent-rich” washing solutionmay vary, but is typically less than the temperature used for the“water-rich” solution. For example, the temperature of the“solvent-rich” solution may be from about 10° C. to about 90° C., insome embodiments from about 15° C. to about 80° C., in some embodimentsfrom about 20° C. to about 60° C., and in some embodiments, from about25° C. to about 50° C. Notably, the use of a lower temperature for the“solvent-rich” solution can minimize the degree to which oligomers areremoved and is economically advantageous as less heating is required. Incertain cases, heating may be conducted at a temperature that is abovethe atmospheric pressure boiling point of a solvent in the mixture. Insuch embodiments, the heating is typically conducted under a relativelyhigh pressure, such as above 1 atm, in some embodiments above about 2atm, and in some embodiments, from about 3 to about 10 atm.

The manner in which the polyarylene sulfide is contacted with the firstand second washing solutions may vary as desired. In one embodiment, forinstance, a system may be employed in which the polyarylene sulfide iscontacted with the washing solutions within a vessel, such as a bath,sedimentation column, etc. Referring to FIGS. 1-3, for instance, oneembodiment of a sedimentation column 10 is shown that is configured toreceive a polyarylene sulfide and washing solution. The sedimentationcolumn 10 may include an upper section 12 that includes liquid outlet20, a middle section 14 that includes inlet 24, and a lower section 16that includes a solids outlet 22 and a liquid inlet 26. Thoughillustrated with a vertical arrangement, it should be understood thatthe sedimentation column may be utilized at other than a verticalarrangement, and the sedimentation column may be at an angle to verticalas long as solids flow through the sedimentation column from the inlet24 to the outlet 22 may be by gravitational force.

The upper section 12 and the lower section 16 may have cross sectionalareas that are larger than that of the middle section 14. In oneembodiment, the sections of the sedimentation column 10 may be circularin cross section, in which case the upper section 12 and lower section16 may have cross sectional diameters that are larger than the crosssectional diameter of the middle section 14. For instance, the uppersection 12 and the lower section 16 may have diameters that are fromabout 1.4 to about 3 times greater than the diameter of the middlesection. For instance, the upper and lower sections may independentlyhave diameters that are about 1.4, about 2, or about 2.5 times greaterthan the diameter of the middle section 14. The larger cross sectionalarea of the upper section 12 may prevent overflow of solids at theoutlet 20 and the larger cross sectional area of the lower section 16may prevent solids flow restriction at the outlet 22. It should beunderstood that the sedimentation column 10 is not limited to anyparticular geometric shape and the cross section of the sedimentationcolumn is not limited to circular. Moreover, the cross sectional shapeof each section of the sedimentation column may vary with regard to oneanother. For example, one or two of the upper section 12, the middlesection 14, and the lower section 16 may have an oval-shaped crosssection, while the other section(s) may be round in cross section.

The middle section 14 of the sedimentation column 10 may include aninlet 24 through which a polymer slurry may be fed to the sedimentationcolumn 10. The slurry may include the polyarylene sulfide in conjunctionwith other byproducts from the formation process, such as reactionsolvents (e.g., N-methylpyrrolidone), salt byproducts, unreactedmonomers or oligomers, etc. As shown in FIG. 2, the inlet 24 meets awall 25 of the middle section 14 substantially tangent to the wall 25.The term “substantially tangent” as utilized herein may be determined bythe distance between the true tangent of the wall 25 of the middlesection 14 and the outer wall 27 of the inlet 24. When the inlet 24meets the wall 25 of the middle section at a perfect tangent, thisdistance will be 0. In general, this distance will be less than about 5centimeters, for instance less than about 3 centimeters. Placement ofthe inlet 24 such that the inlet 24 is substantially tangent to theouter wall 25 of the middle section 14 may prevent disruption of thefluid flow pattern within the sedimentation column 10. This may improvecontact and mass transfer between the downward flowing solids and theupward flowing liquid and may also prevent loss of solids through theoutlet 20 of the upper section 12. To further ensure that solids are notlost through the outlet 20, the inlet 24 may be placed in the middlesection 14 at a distance from the junction 23 where the middle section14 meets the upper section 12. For instance, the vertical distancebetween the midpoint of the inlet 24 and the junction 23 may be equal toor greater than about 5% of the total height of the middle section 14.For instance, the vertical distance between the midpoint of the inlet 24and the junction 23 may be from about 5% to about 50% of the totalheight of the middle section 14. The total height of the middle section14 is that distance between the junction 23, where the upper section 12meets the middle section 14 and the junction 21, where the middlesection 14 meets the lower section 16.

The line at inlet 24 may carry the slurry from a polymerization reactionapparatus to the middle section 14 of the sedimentation column 10. Themiddle section 14 of the sedimentation column may include an agitator 30that incorporates an axial shaft 31 and a series of stirring blades 32along the axial length of the middle section 14. The agitator 30 mayminimize channeling of liquid within the sediment (fluidized bed) andmay maintain contact between the slurry contents and the upwardlyflowing solvent as well as maintain flow of the solids through thesedimentation column 10. The stirring blades 32 may extend from theaxial shaft 31 toward the wall 25 of the middle section 14. In general,the stirring blades may extend at least half of the distance from theaxial shaft to the wall 25 and, in one embodiment, may extend almost allof the way to the wall 25. In one embodiment, the sedimentation columnmay be free of sedimentation plates or trays as have been utilized inpreviously known sedimentation columns.

As shown, the axial shaft 31 may support a series of stirring blades 32along the length of the middle section 14. In general, at least twostirring blades 32 may extend from the axial shaft 31 in a balancedarrangement at each point of blade extension. This is not a requirement,however, and three, four, or more stirring blades may extend from theaxial shaft 31 at a single location along the shaft 31 or alternatively,a single blade may extend from a single location on the shaft 31 and thestirring blades may be offset from one another as they pass down thelength of the shaft 31 so as to maintain balance of the agitator 30during use. The axial shaft 31 may have stirring blades 32 extendingtherefrom at a plurality of locations along the shaft 31. For instance,the axial shaft may have stirring blades extending therefrom at fromabout 3 to about 50 locations along axial shaft 31, with two or morestirring blades 32 extending from the axial shaft at each location. Inone embodiment, the distribution of the blades along the axial shaft 31may be such that there are more blades in the fluidized bed section atthe bottom as compared to the number of blades in the upper portion ofsection 14. During operation, the axial shaft 31 may rotate at a speedthat is typically from about 0.1 rpm to about 1000 rpm, for instancefrom about 0.5 rpm to about 200 rpm or from about 1 rpm to about 50 rpm.

In the illustrated embodiment, a countercurrent flow is employed inwhich the polymer slurry flow is in a direction opposite to that of theflow of the washing solution. Referring again to FIG. 1, for instance,the polymer slurry is fed to the middle section 14 of the sedimentationcolumn 10 via the inlet 24. A washing solution is, on the other hand,fed to the lower section 16 of the column 10 via an inlet 26. In thismanner, the washing solution can flow upwardly through the column as itcontacts the polymer slurry as it flows downwardly through the columntowards the solids outlet 22. If desired, the inlet 26 may include adistributor 35 that may enhance fluid flow through the solids andprevent solids from entering the inlet 26. The lower section 16 may alsohave a conical shape to concentrate the solids content at the outlet 22.The solids content of the slurry at the outlet 22 may generally be about20 wt. % or greater, or about 22 wt. % or greater in some embodiments.If desired, a washing solution may be heated prior to being fed to theinlet 26, such as described above. In such embodiments, thesedimentation column may include heating elements to maintain anelevated temperature during the washing process.

If desired, a fluidized bed may also be formed in the sedimentationcolumn with increasing concentration of solids from the top of the bedto the solids outlet 22. The fluidized bed height may be monitored andcontrolled so as to better control the residence time of solids in thesedimentation column. Through improved control of residence time for thesedimentation column, the efficiency of the separation process carriedout within the sedimentation column may be improved, which may translateto lower operational costs and improved separations. In addition,control of the fluidized bed height and residence time may help toprevent solids loss through the liquid outlet 20 of the upper section12. A sensor may be used to monitor the fluidized bed height within thesedimentation column 10. The sensor type is not limited and may be anysuitable sensor that may monitor the fluidized bed height including bothinternal sensors and external sensors. For example, sensors may utilize,without limitation, optical, infrared, radio frequency, displacement,radar, bubble, vibrational, acoustic, thermal, pressure, nuclear and/ormagnetic detection mechanisms to determine the fluidized bed heightwithin the sedimentation column 10. By way of example, in oneembodiment, an optical sensor 40 (e.g., a laser-based sensor including alaser source and a detector) may be located within the middle section 14of the sedimentation column 10, for instance near the level of the inlet24, which may detect reflection of the laser to determine relativedensity differences of the materials within the sedimentation column 10and thus convey information concerning the location of the top of thefluidized bed to a control system. The control system may relay thatinformation to a valve that may control flow of solids out of thesedimentation column at outlet 22 and/or a valve that may control flowof solids into the sedimentation column at inlet 24 so as to control thebed height. Surge tanks may also be included in the lines leading to andfrom the sedimentation column as is known to maintain control of thefluidized bed height. Other systems as are known in the art forcontrolling the height of a fluidized bed may alternatively be utilized,and the method and system utilized to control the bed height is notparticularly limited. The top of the fluidized bed may be at or near theinlet 24. To improve control of the residence time of the solids in thesedimentation column 10, the sediment bed height variation during theprocess may vary less than about 10% of the total height of the middlesection 14. For instance, the fluidized bed height variation during aprocess may be less than about 5% of the total height of the middlesection 14.

Though illustrated in FIG. 1 as a cylindrical column, the longitudinalcross-sectional shape of the middle section 14 is not limited to thisembodiment. For example, and as illustrated in FIG. 3, middle sections14 a, 14 b, and 14 c of sedimentation columns may be straight or taperedwith increasing angles from a cylindrical middle section as illustratedat 14 a to increasing angles as shown at 14 b and 14 c. When tapered,the middle section may be wider at the top than at the bottom so as toincrease solids concentration at the bottom of the sedimentation columnwithout impeding transport of solids. That is, if the angle of taper istoo large, solids flow may be impeded at the wall of the middle section.A preferred taper angle will vary for each system depending upon flowrates, physical properties of the compounds to be carried within thesystem such as particle size and shape, as well as depending on columnmaterials and surface roughness.

The first and second washing stages may be performed within a singleapparatus (e.g., sedimentation column) such as shown in FIGS. 1-3. Insuch embodiments, the polyarylene sulfide may be initially contactedwith the first washing solution via the inlet 26. The first washingsolution may flow through the column in a direction counter to that ofthe polymer slurry until reaching the outlet 20, where the solution isremoved. Thereafter, the polyarylene sulfide may be contacted with thesecond washing solution via the inlet 26. The second washing solutionmay also flow through the column in a direction counter to that of thepolymer slurry until reaching the outlet 20. Of course, in alternativeembodiments, multiple sedimentation columns may be used in series, oneor more of which have a countercurrent flow such as described above. Inone embodiment, for instance, the first washing solution may be suppliedto a first sedimentation column where it flows in a direction counter tothat of the polymer slurry until reaching an outlet. A solids outlet maythereafter feed solids from the first sedimentation column to a slurryinlet of a second sedimentation column. A second washing solution willmay then flow through the second column in a direction counter to thatof the solids until reaching an outlet.

During the washing process, the polyarylene sulfide is typicallyseparated from the washing solution (e.g., within a countercurrentwashing apparatus). If desired, however, additional separationtechniques may also be employed, such as vibratory screening, etc.

III. Heat Treatment

Regardless of the particular washing technique employed, the washed andoptionally separated polyarylene sulfide may be subjected to a heattreatment to help reduce the volatile content. As noted above, the heattreatment generally occurs in the presence of an inert gas and at arelatively high temperature, such as from about 150° C. to about 275°C., in some embodiments from about 180° C. to about 270° C., and in someembodiments, from about 225° C. to about 265° C. If desired, the inertgas may be recycled. As a result of this treatment, the polyarylenesulfide may have a relatively low volatile content and Yellowness Indexas indicated above. Although by no means required, it is sometimesdesirable to subject the polyarylene sulfide to a drying step prior tothe heat treatment. When performed, the drying step is typicallyconducted at a relatively low temperature, such as from about 80° C. toabout 150° C., in some embodiments from about 90° C. to about 145° C.,and in some embodiments, from about 100° C. to about 140° C. If desired,drying may occur in an ambient atmosphere (e.g., air) or in the presenceof an inert gas such as described above.

The present invention may be better understood with reference to thefollowing example.

Test Methods

Molecular Weight: A sample of PPS may be initially converted to PPSO byoxidation with a mixture of cold HNO₃ (50%) in a trifluoroacetic acidmixture. The resulting PPSO may be dissolved in warmhexafluoroisopropanol (HFIP) for 1 hour and then analyzed for molecularweight by GPC equipped with PSS-hexafluoroisopropanol (HFIP) gelcolumns. The gel columns may be fitted with an HFIP-gel guard columnusing HFIP as mobile phase and refractive index (RI) as detector.

Melt Viscosity: The melt viscosity may be determined as scanning shearrate viscosity and determined in accordance with ISO Test No. 11443:2005(technically equivalent to ASTM D3835-08) at a shear rate of 1,200 s⁻¹and at a temperature of about 310° C. using a Dynisco 7001 capillaryrheometer. The rheometer orifice (die) may have a diameter of 1 mm, alength of 20 mm, an L/D ratio of 20.1, and an entrance angle of 180°.The diameter of the barrel may be 9.55 mm+0.005 mm and the length of therod was 233.4 mm. Prior to measurement, samples are dried in a vacuumoven for 1.5 hours at 150° C.

Crystallization Temperature: The crystallization temperature may bedetermined by differential scanning calorimetry (“DSC”) as is known inthe art. Under the DSC procedure, samples are heated during a firstheating cycle at a rate of 50° C. per minute to a temperature of 340°C., cooled at a rate of 10° C. per minute to a temperature of 50° C.,and then heated during a second heating cycle at a rate of 50° C. perminute to a temperature of 340° C., and cooled again at a rate of 10° C.per minute to a temperature of 50° C. using DSC measurements conductedon a TA Q2000 Instrument. The temperature at the highest point of theexothermic curve obtained during the second heating cycle is generallyreferred to herein as the “crystallization temperature.”

Oligomer Content: The oligomer content of a sample may be determined bycontacting the sample with an extraction solution that contains 100 wt.% chloroform at a temperature of 60° C. and pressure of 1,500 psi. Thesample is rinsed twice with the extraction solution, and thereafter theextracted solvent is dried and the weight of the extractables ismeasured. The oligomer content is determined by dividing the weight ofthe extractables by the weight of the original sample, and thenmultiplying by 100.

Volatile Content: The volatile content may be determined by subjecting asample to a high temperature and then trapping any resulting “outgas”,which is then analyzed by gas chromatography using known techniques.More particularly, 3.0 grams of a dried sample may be placed in a glasstube and then heated to 320° C. for 20 minutes. The off-gas or volatilematerials generated are trapped by means of a cold-trap. Once trapped,the off-gas is analyzed in a solution of acetonitrile by gaschromatography to determine the presence of volatile compounds (e.g.,MO, MMP, BL, DAA, PhSH, MeSPh, pDCB, NMP, etc.). Biphenyl is used as aninternal standard for the analysis.

Residual Ash: To determine the amount of residual inorganic residue(e.g., sodium chloride), an ash test may be performed. Moreparticularly, about 4 grams of a dried sample may be placed in a cleanporcelain crucible with a quartz cover. The crucible is placed in amuffle furnace and heated to 750° C. for 12 hours. After cooling, thecrucible is allowed to cool to room temperature in a desiccator. Theweight is of residual ash is determined.

Yellowness Index: The yellowness index of a sample may be determined inaccordance with ASTM E313-15 (illuminant D65, 10 degree observer).

EXAMPLE

A polyphenylene reaction slurry is made from a process that involves thereaction of NaSH, pDCB, NMP, NaOH, and water. After the polymerizationprocess, substantial amount of the NaCl by-product and NMP are removedfrom the slurry by shaking on a sieve screen (106 microns). The polymeris then re-slurried in acetone containing 200 ppm of MMP and 1500 ppm ofMO (PPS:Acetone by weight=1:5). After mixing for 10 minutes, the slurryis filtered through a 106-micron screen. The acetone re-slurrying stepsare repeated 3 more times. The polymer granules are washed 6 times withwater and dried in an oven under constant nitrogen purge at 105° C. for1.5 hours. During this time, exposure to light is kept to minimum topreserve the initial yellowness index of the flake. The PPS sample isdivided into three portions. Two of the portions are then subjected toheat treatment under air or nitrogen. To perform the heat treatment,about 4.5 grams of the dried PPS flakes are disposed in a 0.5-inch ODglass tube, which is then placed in an aluminum heating block maintainedat 250° C. The heat treatment under air is conducted for 3 hours and theheat treatment under nitrogen is conducted for 3 hours by connecting afeed of slow flow nitrogen. After cooling, the samples are then analyzedfor volatiles content. The results are shown in Table 1 below.

TABLE 1 Without With Heat With Heat Heat Treatment Treatment Volatiles(in mg/kg) Treatment Under Air under N₂ Mesityl oxide (MO) 174 Notmeasured <1 sample degraded Diacetone alcohol (DAA) 60 Not measured <1sample degraded Thiophenol (PhSH) 4 Not measured <1 sample degraded4-Mercapto-4-methyl-penta- 22 Not measured <1 2-one (MMP) sampledegraded Butyrolactone (BL) 390 Not measured 62 sample degradedp-Dichlorobenzene (pDCB) 43 Not measured <1 sample degradedMethylthiophenol (MeSPh) 0 Not measured <1 sample degradedN-Methylpyrrolidone (NMP) 14 Not measured <1 sample degraded TotalVolatile Content 709 Not measured 62 sample degraded

The yellowness index was also determined and was 6.25 without heattreatment, 53.31 with heat treatment under air, and 9.49 with heattreatment under nitrogen.

While particular embodiments of the present disclosure have beenillustrated and described, it would be apparent to one skilled in theart that various other modifications may be made without departing fromthe spirit and scope of the disclosure. It is therefore intended tocover in the appended claims all such changes and modifications that arewithin the scope of this disclosure.

What is claimed is:
 1. A method for forming a polyarylene sulfide, themethod comprising subjecting a polyarylene sulfide to a washing cycle toform a washed polyarylene sulfide, and thereafter subjecting the washedpolyarylene sulfide to a heat treatment in the presence of an atmospherethat includes an inert gas, wherein the heat treatment occurs at atemperature of from about 150° C. to about 275° C.
 2. The method ofclaim 1, wherein the heat treatment occurs for a time period of fromabout 0.1 to about 15 hours.
 3. The method of claim 1, wherein the inertgas includes nitrogen, helium, argon, xenon, neon, krypton, radon, or acombination thereof.
 4. The method of claim 1, wherein inert gasesconstitute from about 90 wt. % to 100 wt. % of the atmosphere.
 5. Themethod of claim 1, wherein the inert gas is recycled.
 6. The method ofclaim 1, wherein after being subjected to the heat treatment, thepolyarylene sulfide has a volatile content of about 175 parts permillion or less.
 7. The method of claim 1, wherein after being subjectedto the heat treatment, the polyarylene sulfide has a mesityl oxidecontent of about 75 ppm or less, a butyrolactone content of about 65 ppmor less, an N-methylpyrrolidone content of about 20 ppm or less, and/ora pDCB content of about 4 ppm or less.
 8. The method of claim 1, whereinafter being subjected to the heat treatment, the polyarylene sulfide hasa Yellowness Index of about 15 or less.
 9. The method of claim 1,wherein after being subjected to the heat treatment, the polyarylenesulfide has an oligomer content of from about 0.5 wt. % to about 2 wt.%.
 10. The method of claim 1, wherein the polyarylene sulfide is alinear polyphenylene sulfide.
 11. The method of claim 1, wherein thewashing cycle includes contacting the polyarylene sulfide with a washingsolution.
 12. The method of claim 11, wherein the washing cycle includesa first washing stage and a subsequent second washing stage, wherein thefirst washing stage includes contacting the polyarylene sulfide with afirst washing solution and the second washing stage includes contactingthe polyarylene sulfide with a second washing solution.
 13. The methodof claim 11, wherein the first washing solution contains an organicsolvent in an amount of about 50 wt. % or more and the second washingsolution contains water in an amount of about 50 wt. % or more.
 14. Themethod of claim 13, wherein the second washing solution is at atemperature of about 90° C. or more
 15. The method of claim 13, whereinthe organic solvent includes N-methylpyrrolidone.
 16. The method ofclaim 13, wherein the first washing solution also contains water. 17.The method of claim 11, wherein the washing solution is generally freeof acetone.
 18. The method of claim 11, wherein the polyarylene sulfideis contacted with the washing solution within a sedimentation column.19. The method of claim 11, further comprising separating the washedpolyarylene sulfide from the washing solution prior to the heattreatment.
 20. The method of claim 1, further comprising drying thewashed polyarylene sulfide prior to the heat treatment.
 21. The methodof claim 20, wherein drying occurs at a temperature of from about 80° C.to about 150° C.
 22. The method of claim 1, wherein after beingsubjected to the heat treatment, the polyarylene sulfide has a meltviscosity of from about 100 to about 2,000 poise, as determined inaccordance with ISO Test No. 11443:2005 at a shear rate of 1200 s⁻¹ andat a temperature of 310° C.
 23. The method of claim 1, wherein afterbeing subjected to the heat treatment, the polyarylene sulfide has anumber average molecular weight of from about 20,000 to about 60,000Daltons.
 24. The method of claim 1, wherein after being subjected to theheat treatment, the polyarylene sulfide has a yellowness index of about10 or less, as determined in accordance with ASTM E313-15 (illuminantD65, 10 degree observer).